If I understand you, you run PW and got:
-FPO-1 ... 8.00-8:40
-FPO-2 ... 8:40-9:10
After that, you changed start time of FPO-2 and get:
-FPO-1 ... 8.00-8:15
-FPO-2 ... 8.15-8:45
-FPO-1 ... 8.45-9:10
What start time of FPO-2 you changed? As I see, you want to start with FPO-2 earlier, but you want first finish with FPO-1?
If I'm in right, you cannot change only FPO-2 start date. Mabe I'm wrong and I didn't understood the problem?